1. Field of the Invention
The present invention relates to a method of manufacturing a color electroluminescent display apparatus applied to various types of thin-plate display apparatuses, and a method of bonding light-transmitting substrates used for the same.
2. Description of the Related Art
An electroluminescent (hereinafter, abbreviated as "EL") display apparatus is a thin display apparatus capable of matrix display like a liquid crystal display apparatus. The EL display apparatus comprises a plurality of EL devices arranged on a substrate. Each of the EL devices comprises an EL light emission layer interposed between a pair of electrodes, and acts as a picture element of the EL display apparatus. When a so-called high alternating field is generated between the pair of electrodes, the EL light emission layer causes electroluminescence, so that light is emitted from the EL light emission layer. This light is so-called EL light. That is, unlike the liquid crystal display apparatus, the EL display apparatus is of a self-light-emission-type display apparatus that is constituted only by solid-state devices. Moreover, compared to the liquid crystal display apparatus, the EL display apparatus is high in contrast and excellent in legibility. Since the EL display apparatus has the above-mentioned characteristics that cannot be obtained from the liquid crystal display apparatus, research thereon is being widely performed. In recent years, research has been performed on the achievement of a thin-film EL display apparatus capable of color display. The thin-film EL display apparatus, which is a type of the EL display apparatus, uses so-called thin-film EL devices as the EL devices.
The applicant has proposed a first prior art regarding the achievement of an EL display apparatus capable of color display in Japanese Examined Patent Publication JP-B2 3-77640 (1991). A color EL display apparatus using thin-film EL devices according to the first prior art has a structure such that three kinds of EL light emission layers that emit light beams of wavelengths of red, green and blue by electroluminescence, respectively, are arranged in parallel and the EL light emission layers are each sandwiched between a pair of electrodes. In order to put the color EL display apparatus to practical use, it is necessary for the EL light emission layers to emit light beams of the wavelengths with a brightness necessary for matrix display. Generally, the wavelength and the brightness of the light emitted from an EL light emission layer by electroluminescence depends on the material of the EL light emission layer. Since there are few materials that emit light beams of the wavelengths of red, green and blue with the above-mentioned brightness, it is difficult to realize the color EL display apparatus.
A second prior art regarding the achievement of a color EL display apparatus is disclosed in Japanese Unexamined Patent Publication JP-A 64-40887 (1989). The color EL display apparatus of this prior art includes a plurality of thin-film EL devices of a double insulation structure including a light emission layer that emits so-called white light by electroluminescence, and a plurality of organic color filters each transmitting light of the wavelength of only one of red, green and blue. The color filters are each placed directly on one electrode of the pair of electrodes of each thin-film EL device. The white light emitted from each EL device is divided into spectra by the color filters. Consequently, light beams of the wavelengths of red, blue and green exit from the color EL display apparatus.
The thin-film EL devices of the double insulation structure in which thin-film insulation layers are interposed between the electrodes and the light emission layers has a structure such that a multiplicity of thin film pieces are laminated. In forming the film pieces, a defect such as a pinhole is sometimes caused in the film pieces. In the case where the insulation layer has a defect such as a pinhole, when the high alternating field is created between the pair of electrodes of the thin-film EL devices, an electrical breakdown is caused at the pinhole and in the vicinity thereof, so that a microdischarge is caused. There are cases where the color filters on the electrodes deteriorates and breaks due to the microdischarge.
As a third prior art regarding the achievement of a color EL display apparatus, Japanese Unexamined Patent Publication JP-A 64-40888 (1989) discloses an art to prevent the above-mentioned deterioration and breakage of the color filters. FIG. 20 is an enlarged partial cross-sectional view of a color EL display apparatus 1 according to this prior art. The color EL display apparatus 1 comprises a main substrate 3, a light-transmitting substrate 4, a plurality of thin-film EL devices 5 of the double insulation structure, a plurality of color filters 6 and a sealing portion 7. The thin-film EL devices 5 are arranged on one surface 9 of the main substrate 3. The color filters 6 are arranged on one surface 10 of the light-transmitting substrate 4. The main substrate 3 and the light-transmitting substrate 4 are disposed so that the surfaces 9 and 10 are opposed with a predetermined gap in between. The sealing portion 7, which is so-called passivation protecting means, is disposed between the main substrate 3 and the light-transmitting substrate 4.
The method of manufacturing the color EL display apparatus 1 will briefly be described below. First, a plurality of lower electrodes 11 which are thin film strips are formed on the one surface 9 of the substrate 3. Then, a lower insulation layer 12, a light emission layer 13 and an upper insulation layer 14 are successively laminated in this order on all the lower electrodes 11. Then, a plurality of upper electrodes 15 which are thin film strips are formed on the upper insulation layer 14. The upper electrodes 15 each transmit light. The direction of length of the lower electrodes 11 and the direction of length of the upper electrodes 15 are perpendicular to each other when viewed from the direction of the normal 16 to the one surface 9 of the substrate 3. The portions where the lower electrodes 11 and the upper electrodes 15 intersect when viewed from the direction of the normal 16 are the thin-film EL devices 5. In order to improve the crystallinity of the light emission layer 13, after the light emission layer 13 is formed or after the upper insulation layer 14 is formed, annealing is performed in a vacuum or in an inert gas.
Then, the color filters 6 are formed on the one surface 10 of the light-transmitting substrate 4. Then, the substrate 3 and the light-transmitting substrate 4 are bonded by an epoxy resin 17 so that the surfaces 9 and 10 are opposed with the predetermined gap in between. Lastly, in order to form a protective material layer 18, the gap between the substrate 3 and the light-transmitting substrate 4 is filled with a gaseous or a liquid protective material. The epoxy resin 17 and the protective material layer 18 constitute the sealing portion 7. By the above-described process, the color EL display apparatus is completed.
Generally, the inactivation protecting means, that is, the sealing portion 7 is provided for shielding the thin-film EL devices from the atmosphere to thereby stabilize the thin-film EL devices and protecting the thin-film EL devices from mechanical failures. A so-called seal life which is one of the greatest characteristics of typical EL display apparatuses depends on the structure of the sealing portion.
A sealing portion currently used in a typical thin-film EL display apparatus is formed by use of a sealing substrate and amixture liquid of silica gel and silicone oil. The process of forming the currently-used sealing portion will be described below. The typical thin-film EL display apparatus comprises a plurality of EL devices arranged on one surface of a substrate. First, a concave portion with a depth of 300 to 500 .mu.m is formed in one surface of the sealing substrate. Then, the sealing substrate and the substrate are bonded together so that the concave portion is opposed to the one surface of the substrate and that a filling hole is left. Then, the gap between the substrate and the sealing substrate is evacuated, and the mixture liquid is filled into the gap. Lastly, the filling hole is sealed, which completes the currently-used sealing portion.
The silica gel absorbs moisture intruding into the gap. The silicone oil circulates the silica gel in the gap and cools the EL devices. Consequently, the EL devices are protected from the influence of moisture and the like. The seal life obtained from the currently-used sealing portion is not less than 50 thousand hours.
In the color EL display apparatus 1 of the third prior art, the light-transmitting substrate 4 is used instead of the sealing substrate, and the protective material is filled into the gap between the substrate 3 and the light-transmitting substrate 4 to form the sealing portion 7. However, in the color EL display apparatus 1, in order to ensure a viewing angle sufficient for practical use, it is necessary that the gap between the substrate 3 and the light-transmitting substrate 4 be minimized. Consequently, the gap between the substrate 3 and the light-transmitting substrate 4 is frequently a fraction of the width of the gap in the currently-used sealing portion. Therefore, the seal life of the color EL display apparatus 1 is reduced to a fraction of that of the EL display apparatus having the currently-used sealing portion. Further, in the color EL display apparatus 1, since the silica gel enters the gap between the EL devices and the color filters, blur and distortion are caused in the display.
As a fourth prior art regarding the achievement of a color EL display apparatus, Japanese Unexamined Patent Publication JP-A 64-40888 (1989) further discloses an art to form color filters while employing the currently-used sealing portion. A color EL display apparatus according to the fourth prior art has a structure such that a plurality of EL devices are arranged on one surface of a substrate, a plurality of color filters are arranged on the other surface of the substrate and the above-described currently-used sealing portion is disposed on the one surface of the substrate. However, in the color EL display apparatus of the fourth prior art, since the thickness of the substrate is 1 to 2 mm, color displacements of the display are apt to be large and it is difficult to increase the degree of precision of the thin-film EL devices. Moreover, since the substrate thickness of the EL display apparatus is generally not less than 1.1 mm, the viewing angle of the color EL display apparatus of the fourth prior art is apt to be extremely narrow compared to those of the EL display apparatuses of the first to the third prior arts.
In order to improve the viewing angle of the color EL display apparatus of the fourth prior art, it is necessary that the thickness of the substrate be smaller than the thickness of general substrates. When the thickness of the substrate is reduced, in the steps of forming various thin films and the step of photoprocess in the process of manufacturing the color EL display apparatus of the fourth prior art, it is difficult to ensure a substrate strength necessary for the steps and to handle the substrate. At the same time, there is a possibility that the substrate cracks when the gap is evacuated in order to fill the mixture liquid into the gap. Because of these problems, it is difficult to reduce the thickness of the color EL display apparatus of the fourth prior art so as to be smaller than the thickness of general substrates.
Moreover, there are cases where a photo-setting resin is used, for example, for bonding a light-transmitting substrate having the color filters disposed thereon and the other substrate. However, since the color filters intercept light necessary for hardening the photo-setting resin such as ultraviolet rays, the photo-setting resin cannot harden in the area where the color filters are disposed.
Moreover, in the case where two light transmitting substrates are bonded together by filling the gap between the substrates with an adhesive, when there are portions where the adhesive is absent, that is, when bubbles are formed, the refractive index of the portions is different from that of the surrounding portions, so that the configurations of the bubbles appear on the display screen when a display is provided. This degrades the display quality.
In the case where two light-transmitting glass substrates are bonded together, a conventionally used method is such that an adhesive is thinly applied onto the surface of one glass substrate and then, the two substrates are brought into intimate contact with each other. However, in the case where an adhesive is applied, it cannot be helped that slight wavy patterns are formed on the surface. When the uneven portions of the wavy patterns are in contact with the substrate, air gaps are formed between the substrate and the adhesive. When there is no place for the air in the air gaps to escape into, the air gaps are left as bubbles.
As a method of bonding two substrates without such bubbles being formed, for example, Japanese Unexamined Patent Publication JP-A 63-18326 (1988) discloses a method in which bubbles are blown off by a spinner rotation after the substrates are bonded. Japanese Unexamined Patent Publication JP-A 9-278497 (1997) discloses a method in which when substrates are bonded together, the substrates are inclined by an apparatus for controlling the angles of the substrates in order that no bubbles are formed. Japanese Unexamined Patent Publication JP-A3-126646 (1991) discloses a method in which when an adhesive is applied to the substrate, the thickness of the adhesive is controlled so as to monotonously increase from one end to the other end to thereby prevent the formation of bubbles when the substrates are bonded together. Japanese Unexamined Patent Publication JP-A 6-349962 (1994) discloses a bonding method in which a central portion of glass formed so that the central portion thereof is higher than the other portions thereof is melted to bond the two substrates to thereby prevent the formation of bubbles. However, the bonding methods disclosed in the prior arts all require separate apparatuses having complicated structures, which increases the manufacturing cost. Therefore, an easier method is desired in order to increase industrial use.
Moreover, when two substrates are bonded together by dropping a liquid hardening resin on one surface of a thin substrate, there are cases where the dropped resin concaves the thin substrate and the concaved portion is left as a distortion of the EL display screen to degrade the display quality.
Moreover, glass substrates generally have local and small asperities. FIG. 21 is a plan view schematically showing conditions in bonding substrates having such local concave portions. When the substrates are brought into intimate contact with each other so that a liquid hardening resin spreads between the substrates, as shown in FIG. 21, although the speed at which the resin spreads is constant in the area where a local concave portion P is absent, the resin spreading speed is lower in the local concave portion P than in the periphery. Consequently, as shown in FIG. 21, the resin spreads over the periphery of the concave portion P faster in the vicinity of the concave portion P, so that the periphery is filled with the resin without the resin being spread over the concave portion P. As a result, a bubble is left in the concave portion P.